The present invention relates to a MOSFET device and a method for manufacturing the same, and more particularly, to a MOSFET device which suppresses electrical coupling between adjoining recess gates and a method for manufacturing the same.
As a semiconductor device becomes more highly integrated, the channel length of a transistor decreases. Therefore, when attaining the target threshold voltage of a MOSFET device, limitations necessarily exist in a conventional planar type transistor structure.
In order to overcome the problems caused by the short channel length, a MOSFET device having a three-dimensional gate (e.g., a recess gate) has been developed.
FIG. 1 is a cross-sectional view illustrating a conventional MOSFET device. Referring to FIG. 1, a groove H delimiting a gate area is defined by forming a recess in a semiconductor substrate 100 having an isolation structure 110. A recess gate 130, which includes a gate insulation layer 132, a gate conductive layer 134 and a gate hard mask layer 136, is formed in the groove H. Junction areas 150 are formed in the semiconductor substrate 100 on both sides of the recess gate 130.
In the MOSFET device having the recess gate structure described above, the recessed portion of the semiconductor substrate increases the channel length relative to a planar type MOSFET device.
However, in the MOSFET device having the recess gate structure, the channel length of a transistor is increased by defining a groove. The groove is defined by etching the semiconductor substrate. As a result, the distance between adjoining recess gates is shortened due to the structural characteristic of the groove.
Therefore, as the distance between adjoining recess gates is decreased, electrical coupling occurs between adjoining recess gates. Thus, electric fields of the adjoining recess gates interact with each other. As a result, the threshold voltage of a cell decreases by the voltage applied to an adjoining recess gate.
In the MOSFET device having the conventional recess gate structure, an effective channel length can be increased relative to the planar type MOSFET device. However, due to the electrical coupling between adjoining recess gates caused by the structural characteristic of the groove, current leakage results and the refresh characteristic of a semiconductor device is degraded.